1. Field of the Invention
The invention relates to a method of forming a rigid thermal insulation material. In particular, it relates to a material suited to high temperature applications, which is formed from a mixture of isotropic pitch carbon fibers and a soluble sugar binder, and will be described with particular reference thereto.
2. Discussion of the Art
Thermal insulation materials formed from carbon fibers exhibit excellent resistance to heat flow, even at high temperatures. Commercially available materials are generally produced from a carbon fiber filler, derived from a cotton, rayon or pitch precursor, and a binder, such as a phenolic resin solution, furfuryl alcohol, or insoluble starch. In one method, the binder and fibers are formed into an artifact under vacuum and then heated to high temperatures to carbonize the binder. For example, thermal insulation materials have been prepared by combining 0.35% of carbonized rayon fibers, 0.35% by weight of an insoluble starch, and 99.3% by weight of water, molding under vacuum, and carbonizing at 1000xc2x0 C. The density of the carbonized insulation material ranged from 0.11 to 0.26 g/cm3, compressive strength ranged from 1-10.5 kg/cm2, and thermal conductivity ranged from 0.066 to 0.11 W/m-xc2x0 K at 538xc2x0 C. and from 0.577 to 0.793 W/m-xc2x0 K at 2200xc2x0 C., measured in an argon atmosphere.
In another method, hot pressing is used to form the artifact, followed by carbonization. Thermal insulation materials formed by hot pressing tend to have a higher density than vacuum molded materials, and thus thermal conductivities tend to be higher. For example, a hot pressed composite formed by combining 50% by weight carbonized rayon fibers and 50% by weight phenolic resin binder or starch slurry, hot pressing, and carbonizing to 1350xc2x0 C. had a density of 0.31-0.91 g/cm3.
The rigid mat thus formed is then machined into desired shapes and, optionally, sealed or coated, for example, with a phenolic resin.
For insulation of large furnaces, it is desirable for insulation materials to be readily removable for replacement. Materials formed from conventional fibers, such as rayon fiber, do not generally have a sufficient structural strength to be formed into boards which can be replaced periodically. Additionally, for furnaces which operate at high temperatures, such as induction furnaces used for graphitization, which operate at temperatures of up to about 3200xc2x0 C., an insulation material having a particularly low thermal conductivity and high thermal stability is desired.
Conventional binders, such as phenolic resin solutions and furfuryl alcohol, tend to pose environmental problems and evolve potentially harmful byproducts during conversion of the resin to carbon during processing. It is also difficult to control the amount of binder-derived carbon that is incorporated into the composite.
The present invention provides a new and improved method and insulation material which overcome the above-referenced problems and others.
In accordance with one aspect of the present invention, a method of forming a thermal insulation material is provided. The method includes combining carbon-containing fibers, which include pitch-based carbon fibers, with a binder which includes a soluble sugar to form a mixture. The mixture is formed into a solid preform having a general shape of the thermal insulation material. The preform is heated to a sufficient temperature to carbonize the preform and form the thermal insulation material.
In accordance with another aspect of the present invention, an insulation material is formed by the method described.
In accordance with another aspect of the present invention, a low density thermal insulation article is provided. The article is formed from a mixture of isotropic pitch fibers and a sugar binder which has been heated to a sufficient temperature to carbonize the mixture. The article has a density of from about 0.1 to about 0.4 g/cm3.
In accordance with another aspect of the present invention, a method of providing thermal insulation for a high temperature radiant heat source is provided. The method includes forming an insulation member having a thermal conductivity of less than about 0.4 W/m-xc2x0 K. The forming step includes filtering a mixture which includes isotropic pitch carbon fibers and a sugar solution and heating the filtered mixture to a temperature of at least 900xc2x0 C. to form the member. The member is positioned adjacent the high temperature radiant heat source to insulate the heat source.
An advantage of at least one embodiment of the present invention is that it provides an insulation material having high flexural strength and low thermal conductivity.
Another advantage of at least one embodiment of the present invention is that the binder is environmentally safe, posing fewer disposal problems than conventional organic binder systems.
Another advantage of at least one embodiment of the present invention is that it enables the density and other properties of the insulation material to be adjusted by varying the concentration of soluble sugar in the binder.
Still further advantages of the present invention will be readily apparent to those skilled in the art, upon a reading of the following disclosure and a review of the accompanying drawings.